Add pledge(2) support for OpenBSD

Add pledge(2) support for OpenBSD

pledges for i3:
"stdio rpath unix" for talking to the i3 socket usually in /tmp
"proc exec" for executing programs
"wpath cpath" are needed for the restart-in-place functionality

To make this work, @semarie pointed out that it is sufficient to ensure
that we get physical_mem_bytes only once, namely in init_logging().

pledges for i3-msg:
"stdio rpath unix" are needed for talking to the i3-socket

pledges for i3-nagbar
"rpath getpw" to find the home directory
"wpath cpath" to write the script
"proc exec" to execute the script